In the packaging of integrated circuits, device dies are bonded onto package substrates, which include metal connections that are used to route electrical signals between opposite sides of the package substrates. The device dies may be bonded onto one side of a package substrate using flip chip bonding, and a reflow is performed to melt the solder balls that interconnect the dies and the package substrate.
The package substrates may use organic materials that have high Coefficients of Thermal Expansion (CTEs), such as materials that can be easily laminated. During the bonding process, since the dies and the package substrates have significantly different CTEs, the warpage in the device dies and the package substrates is worsened. For example, the silicon in the dies may have a CTE of 3.2 ppm/° C., while the package substrates may have a CTE between about 17 ppm/° C. and 20 ppm/° C. The warpage in the package substrates may cause irregular joints and/or bump cracks. As a result, the yield of the packaging process is adversely affected.
In conventional packages, when a Chip-on-Chip-on-Substrate (CoCoS) package is formed, a device die is bonded to a package substrate first. An underfill is then dispensed into the gap between the device die and the package substrate, followed by the curing of the underfill. Since the underfill is dispensed through capillary, it may climb onto the top surface of the first device die, which effect is referred to as underfill overflow. The underfill may also spread far away from the device die, which effect is referred to as underfill bleeding. Both underfill overflow and underfill bleeding cause reliability problems of the resulting package.
In addition, the conventional CoCoS packaging also faces problems since the package substrate and the first device die may warp after their bonding. This poses problems for bonding additional dies onto the device die. Conventionally, the metal lid that is attached to the top surface of the die stack also includes a skirt portion extending down to encircle the die stack, wherein the skirt portion is attached to the package substrate through an adhesive. The metal lid thus has the function of reducing warpage of the package substrate.